The present invention generally relates to a method and apparatus for processing digital images. More particularly, the present invention is directed to identifying interfaces between black and color areas and between printed and non-printed areas.
Liquid ink printers of the type often referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, employ at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
Liquid ink printers including ink jet printers deposit black and/or colored liquid inks which tend to spread when the ink is deposited on paper as a drop, spot, or dot. A problem of liquid ink printers is that the liquid inks used have a finite drying time, which tends to be somewhat longer than desired. Bleeding tends to occur when the drops are placed next to each other in a consecutive order or in a cluster of dots within a short time. Bleeding, spreading, and feathering causes print quality degradation including color shift, reduction in edge sharpness and solid area mottle which includes density variations in said areas due to puddling of inks. Intercolor bleeding occurs when ink from one color area blends into or bleeds with ink from another color area. Intercolor bleeding is often most pronounced where an area of black ink (relatively slow drying) adjoins an area of color ink (relatively fast drying); however, intercolor bleeding can occur at the interface between areas of any color inks having substantially different properties such as dry time or permeability.
Various methods have been proposed to increase edge sharpness and to reduce intercolor bleeding. Some of the proposed methods include replacing slow drying black ink with a process or composite black formed by combing fast drying color inks; under-printing a portion of the slow drying black ink with a color ink, use a fast drying black ink, and using both fast dry and slow dry black ink. While all of the proposed methods reduce intercolor bleeding to some degree, they all have one or more drawbacks that effect printer performance and/or image quality.
For example, using a fast dry ink in place of a slow drying black ink results in a reduced quality of black reproduction as current fast drying black inks have lower image quality than slow drying black inks. Additionally, fast drying black inks typically result in fuzzy edges in black areas next to printed areas. The use fast drying black ink at an interface and slow drying black ink for interior regions can eliminate lower image quality associated with fast drying black inks, but increases the cost and complexity of printer design by requiring a fifth ink in addition to the cyan, magenta, yellow and slow drying black ink. Similarly, replacing slow drying black ink with a process black (composite black) generated from fast drying color inks typically results in a reduced quality of black reproduction resulting in a lower image quality than the use of slow drying black ink. Additionally, the use of process black increases the amount of ink deposited on the print medium, increases dry time and increase the time to print a document. Furthermore, the use of additional ink may not be suitable for print medium such as transparencies and some types of paper which is not very absorbent. Under-printing a portion of the slow drying black ink with a color ink c an be used to reduce intercolor bleeding; however, under-printing increases the amount of ink on the print medium. Moreover, printing color under black often results in the thickening or blurring of edges particularly along edges between printed and non-printed areas.
One aspect of the present invention is a process for processing color image data to determine if a target pixel is within a border region near an interface. The process identifies a target pixel within the color image data; determines whether the target pixel is a black only pixel and, if so, collects a first set of pixel statistics from a first window comprising the target pixel and pixels surrounding the target pixel and determines whether the target pixel is a color only pixel and, if so, collects a second set of pixel statistics from a second window comprising the target pixel and pixels surrounding the target pixel. Additionally, the process analyzes the first set of statistics to determine if the target pixel is within an N-pixel wide black border region near an interface when the target pixel is a black pixel and analyzes the second set of statistics to determine if the target pixel is within an M-pixel wide color border region near an interface when the target pixel is a color only pixel.
Another aspect of the present invention is a device for identifying a border region near an interface between a black area and a color area. The device includes a black statistics collection filter connected to receive a target pixel and a set of surrounding pixels that generates a black statistics signal; a color statistics collection filter connected to receive the target pixel and a set of neighboring pixels that generates a color statistics signal; and a pixel identification circuit connected to receive the black statistics signal and the color statistics signal that generates a pixel identification signal for the target pixel based upon the statistics signals.
A third aspect of the present invention is an inkjet printing system. The inkjet printing system includes an image source providing color image data; a device for reducing intercolor bleeding coupled to receive the color image data, the device further including including a black statistics collection filter connected to receive a target pixel a set of surrounding pixels, the black statistics collection filter generating a black statistics signal, a color statistics collection filter connected to receive the target pixel a set of neighboring pixels, the color statistics collection filter generating a color statistics signal, a pixel identification circuit connected to receive the black and color statistics signals, the identification circuit generating a pixel identification signal for the target pixel, and a pixel modification circuit connected to receive the pixel identification signal, the modification circuit modifying the target pixel in response to the identification signal; and an inkjet printer connected to receive the modified target pixel, the printer generating an output on a suitable medium in response to the modified target pixel.